While the subject invention will be described in connection with the manufacture of exceptionally sharp scalpels, it will be appreciated that the invention discussed herein relates to any cutting implement, the sharpness of which and the durability of which is of importance. As such, the cutting implements described herein include microtomes, razor blades, and knives as well as surgical scalpels.
The best blades produced by current metallic technology have an edge radius of curvature of approximately 500 Angstroms. Diamond blades have been produced which have an edge radius of curvature of approximately 400 Angstroms. More recently, ruby blades have been produced with an edge radius of curvature of 350 Angstroms. All of the above blades have been produced by conventional lapping procedures.
While U.S. Pat. Nos. 2,070,281; 2,838,049; 2,898,906; 3,636,955; 3,844,272; and 3,945,117 describe a number of surgical instruments involving cutting edges or puncturing tools, their manufacture by the techniques described therein produce instruments whose wear characteristics, sharpness and blade drag present problems to surgeons and those other personnel who employ them. In this context the term blade drag refers to the force which resists the movement of the blade in a cutting procedure. The amount of tissue damage during surgery is directly dependent upon the sharpness of the blade and other factors contributing to blade drag, with the amount of extraneous tissue damage determining healing time, as well as the extent of scarring which can accompany the surgical operation.
More particularly, with respect to radial keratotomy which is a procedure in which radial cuts are made about the periphery of the cornea, it is imperative that the cuts made have smooth and flat side walls to reduce the amount of refraction and scattering caused by uneven side walls. Scalpels and other cutting instruments to date, either because of blade drag or because of a lack of sharpness, produce ragged side walls at least to the extent that after surgery, patients experience a diffuse glare looking into a strong light. The ability to provide a scalpel which can produce smooth, flat side walls during such a procedure is therefore of paramount importance.
With respect to microtome applications it will be appreciated that the microtome is an instrument which provides slices of tissue embedded in a carrier. It is of paramount importance especially in electron microscopy to produce microtome samples as thin as possible. In both electron microscopy and optical microscopy, it is of importance that the distortions in the sample due to cutting be minimized, with the distortions again being a function of blade sharpness and other factors contributing to blade drag. The minimum thickness of a substantially undistorted slice is of course directly a function of blade sharpness.
With respect to scalpels and other cutting instruments made of metal, with current technology the wear characteristics are such that the blades dull significantly during use and, more particularly, during a single surgical operation. When delicate surgery is required, the ability to maintain a constant sharpness cutting edge is important to the success of the delicate surgery, with the sharper the cutting edge the better.
It will be appreciated that aluminum oxide, ruby and sapphire are all closely related materials and vary only in the concentrations of dopant metal oxides. All of these materials have the same crystal structure and as such show great simularity in physical behavior. For the purposes of this invention these materials will be referred to as aluminum oxide.
It should be pointed out that in the past, aluminum oxide blades have been utilized for surgical instruments. These blades of sapphire or ruby have in the past been made by lapping techniques which in general produce blades whose drag characteristics and sharpness, while exceeding that associated with metallic blades, nonetheless are nonoptimal for the applications described above. In addition, it will be appreciated that, in all lapping procedures, material is removed from the blade edge mechanically through abrasion, which leaves near-surface damage in the region of the bevel and edge which increases the propensity for wear and fracture.
While not used for single crystalline blades, preferential etching techniques have in the past been utilized in the semiconductor industry for the etching of aluminum oxide and silicon. Chemical polishing in the form of etching has also been utilized in material science in a wide spectrum of applications other than for the production of extremely sharp edges for cutting implements made from single crystal material.
An exception to the above is described in U.S. Pat. No. 4,124,698 in which monocrystalline ribbon produced by a so-called "EFG" process through the utilization of a die is provided with sharpened edges through the utilization of a solvent in the form of a melt of vanadium pentoxide or potassium tantilate niobate by moving a blank of the ribbon through a melt or body of solvent in a direction generally normal to the plane of the ribbon. As mentioned in this patent, the degree of edge uniformity is dependent upon the movement of the ribbon through the solvent to achieve laminar flow, with laminar flow depending upon many process variables, including the shape of the ribbon, the crucible utilized and the viscosity of the solvent. As described in this patent, unidirectional movement oftentimes results in dissolving away portions of the ribbon along only one side. It is said that it is generally preferable to reciprocate the ribbon in the solvent in order to provide for the required edge.
In this patent it is preferred that the ribbon be of a type which has the crystallographic c-axis disposed parallel to its longitudinal geometric axis, since such ribbons exhibit the best edge sharpening when subjected to the method of sharpening described in the patent. In this patent, it will be appreciated that the c-axis lies in the plane of the blade whereas, as will be described hereinafter, in the subject method the c-axis is perpendicular to the plane of the blade and no relative movement of the blade and solvent is required or even desirable.
U.S. Pat. No. 3,894,337 describes the formation of blades, and more particularly a razor blade having a single crystal of .alpha.-aluminum oxide for the cutting edge thereof. This blade is formed by grinding or etching single crystals of aluminum oxide. The crystals are grown preferably in a shape corresponding to the desired cross section of the blade to facilitate formation of the cutting edge. However, in the above-mentioned patent no specific procedure or orientation for single crystals of .alpha.-aluminum oxide is described other than to say that it is convenient to produce cutting edges substantially parallel to the c-axis of the unit cell as opposed to perpendicular to the c-axis, which is one of the aspects of the subject invention to be described. Moreover, a number of etchants are listed in this patent which, for a variety of reasons, do not provide for pit-free chemically polished surfaces over a wide range of crystallographic orientations due to difficulty in handling, temperature processing constraints and physical limitations of the etch-material system. It will be appreciated that the selection of a particular etchant is critical with respect to chemical polishing and the prevention of pitting. It is therefore advantageous to select an etchant which polishes over a large range of crystallographic orientations for the formation of cutting tools.
It is also important to note that U.S. Pat. No. 3,894,337 omits mention of the utilization of a combination of sulfuric acid and phosphoric acid as an etchant for the production of blades. As will be discussed, the combination of these acids along with certain physical constraints is important to the subject invention.
As discussed in the Journal of the Electrochemical Society, October, 1971, Vol. 118, No. 10, sapphire and more recently MgAl spinnel (MgAl.sub.2 O.sub.4) have been employed extensively as single crystal dielectric substrates for the epitaxial growth of a variety of materials. Wafers of these dielectrics are usually available only in mechanically polished form. Because of the damage introduced during mechanical preparation, many and varied liquid and gas phase treatments have been applied prior to use, to effect non-selective etching or chemical polishing of the surfaces. Among these are included molten borax, heated phosphoric acid, molten V.sub.2 O.sub.5, molten lead fluoride, heated vaporous sulfur fluorides, heated vaporous fluorinated hydrocarbons, HCL-H.sub.2 at elevated temperatures and H.sub.2 annealing among others. As described in this article, none of the above-mentioned approaches has proved to be satisfactory for a variety of reasons. Molten salt approaches are cumbersome or often are somewhat selective, and in common with others noted above are highly orientation dependent. Vapor phase etchants are quite orientation dependent and sometimes exhibit low removal rates. Heated phosphoric acid is said to have shown some promise but tends to leave insoluble residues behind. Further, because it dehydrates and polymerizes, its chemical behavior is said to vary continuously with time at temperature. H.sub.3 PO.sub.4 is also said to be somewhat orientation selective and frequently results in dense pitting. Annealing in H.sub.2 exhibits a vanishingly low removal rate and was found not to eliminate polishing scratch damage when subjected subsequently to the liquid phase portion of the polishing technique. However, the use of a combination of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4 is utilized as the preferred etchant in this paper.
The content of this paper was utilized extensively in U.S. Pat. No. 3,964,942. In this patent the polishing of single crystal dielectrics is described utilizing a mixture of sulfuric and phosphoric acid in which polishing is achieved for sapphire having certain crystallographic orientations not optimal to the formation of cutting instruments. It will be appreciated that nowhere in this patent is mentioned the formation of edges for cutting implements, the primary purpose of the patent being directed to the formation of crystal wafers of aluminum oxide or magnesium aluminum spinnel with damage free surfaces.
Other patents describing the etching of sapphire include U.S. Pat. No. 4,052,251 in which sapphire is etched utilizing sulphur hexafluoride. Moreover, U.S. Pat. No. 3,808,065 describes a method of polishing sapphire and spinnel utilizing molten borax. As described above there are certain problems with the utilization of molten borax as an etchant. Finally, with respect to chemically polishing, U.S. Pat. No. 3,878,005 discusses the use of lead monoxide and boric anhydride at reasonably high temperatures of 1100.degree. C. to 1200.degree. C. for the chemical polishing of a variety of different ceramic materials.
With respect to the etchants utilized for etching aluminum oxide, reference is made to a paper entitled "Surface Preparation of Ceramic Oxide Crystals: Work Damage and Microhardness" by Michael F. Ehman, published in the Journal of Electrochemical Society, September, 1974, pp. 1240-1242, in which a combination of sulfuric acid and phosphoric acid is utilized. The thrust of this article is to describe the measurement of the depth of near-surface damage for each crystal as a function of surface orientation and the type of abrasive surface preparation techniques. Moreover, in the Journal of Material Science, Vol. 16, 1981, pp. 1071-1080, in an article entitled "Thermochemical Dissolution of Corundum" published by A. E. Smirnov et al. of the Institute of Crystallography, U.S.S.R. Academy of Sciences, a number of etchants are described. However, none of these etchants include the utilization of a combination of sulfuric acid and phosphoric acid. In accordance with RCA Review, Vol. 34, December, 1973, in an article entitled "The Chemical Polishing of Sapphire and Spinnel", P. H. Robinson and R. O. Wance describe the utilization of molten borax for polishing both sapphire and spinnel substrate orientations that are used to produce (100) oriented epitaxial silicon.
In an article by W. J. Alford and D. L. Stevens, entitled "Chemical Polishing and Etching Techniques for Al.sub.2 O.sub.3 Single Crystals", published in the Journal of the American Ceramic Society, April, 1963, an etching procedure for revealing dislocations intersecting the (0,0,0,1) plane and planes near (2,0,2,1) in ruby and in sapphire is discussed. The surfaces to be etched were prepared by mechanical polishing and subsequent flame polishing. In this article, phosphoric acid alone was utilized, which when utilized alone without sulfuric acid, does not produce a large crystallographic range of chemical polishing necessary for the production of a wide variety of cutting instruments. Moreover, when utilizing this acid alone, the properties of the acid change with time.
In summary, what can be seen from the large amount of literature cited above is that a great many etchants including sulfuric and phosphoric acids have been utilized for the chemical cleaning and polishing of substrates to be utilized in the manufacture of semiconductor products or for the documentation and measurement of dislocation densities in ceramic materials. For the two patents mentioned which utilize monocrystalline material for the formation of cutting instruments, either lapping is utilized in the formation of the edges or chemical etching procedures are described which are difficult to control and which do not utilize a combination of sulfuric acid and phosphoric acid. Moreover, neither of these two patents utilize etchants in a manner consistent with optimal orientation of the crystallographic system for that etchant.